This document is a project report submitted by Kevin V. Alex to the Department of Physics at Sacred Heart College in partial fulfillment of the requirements for a Master of Science degree in physics from Mahatma Gandhi University. The project involved growing and optimizing aluminum-doped zinc oxide thin films using the spray pyrolysis technique under the guidance of Dr. M.K. Jayaraj at the Centre for Advanced Materials, Department of Physics, Cochin University of Science and Technology. Characterization of the aluminum-doped ZnO films was carried out to study the effect of aluminum doping and deposition parameters like growth rate and temperature.

1. GROWTH AND OPTIMIZATION OF Aluminium
DOPED zinc oxide using spray pyrolysis
technique
A project work by
KEVIN.V.ALEX
REG NO: 14PPHY8557
Under the guidance of
Dr. M.K.JAYARAJ
Department of Physics
Cochin University of Science and Technology, Kalamasserry
Submitted to Mahatma Gandhi University in partial fulfillment of the
requirement for the award of the degree of Master of Science in physics
2014-2016
DEPARTMENT OF PHYSICS SACRED HEART COLLEGE,
THEVARA, COCHIN-13

2. Growth and Optimization of Al-doped ZnO using Spray
Pyrolysis technique
PROJECT REPORT
Submitted by Kevin.V.Alex, Reg. No: 14PPHY8557 to the Department of
Physics, Sacred Heart College, Thevara on ………………………… for the approval by
the Department and submitted for the viva voce held on ………………………..at
Sacred Heart College, Thevara
Internal Examiner External Examiners
1. 1.
2.

3. SACRED HEART COLLEGE
THEVARA, COCHIN-682013
(Affiliated to M.G. University)
DEPARTMENT OF PHYSICS
CERTIFICATE
This is to certify that Kevin.V.Alex, Reg.No: 14PPHY8557 has satisfactorily
completed the project entitled ‘Growth and optimization of Al-doped ZnO
using Spray Pyrolysis technique’ at the Centre for Advanced Materials,
Department of Physics, Cochin University of Science and Technology during the
academic year 2014-2016 in partial fulfillment for the award of degree of
Master of Science in Physics from M.G. University, Kottayam.
Dr. SIBY MATHEW Dr. GEORGEKUTTY JOSEPH
Class in charge H.O.D
.

4. COCHIN UNIVERSITY OF SCIENCE AND
TECHNOLOGY
DEPARTMENT OF PHYSICS
CERTIFICATE
This is to certify that the report entitled ‘Growth and Optimization of Al-
doped ZnO using Spray Pyrolysis technique’ is an authentic record of work
done by Kevin.V.Alex, Reg.No: 14PPHY8557 of Sacred Heart College, Thevara,
in the Centre for Advanced Materials, Department of Physics, Cochin University
of Science and Technology, Kalamasserry under my guidance and co-operation
during the academic year 2014-2016 in partial fulfillment to award the degree
of Master of Science in Physics.
Dr. M.K.Jayaraj
Professor
Department of Physics, CUSAT
(Project Guide)

5. DECLARATION
I, KEVIN.V.ALEX hereby declare that this report is a bona-fide record of the
project entitled ‘Growth and optimization of Al-doped ZnO using Spray
Pyrolysis technique’. The study has been undertaken in partial fulfillment for
the award of degree of Master of Science in Physics with specialization in
Advanced Electronics during the academic year 2014-16 under the supervision
of Dr.M.K.JAYARAJ, Centre for Advanced Materials, Department of Physics,
Cochin University of Science and Technology, Kalamasserry.
Place: Thevara KEVIN.V.ALEX
Date:

6. ACKNOWLEDGEMENT
While submitting the project, I acknowledge my deep gratitude to my project guide Dr.
M.K.JAYARAJ and co-guide Dr.R.SREEKUMAR, Centre for Advanced Materials, Department
of Physics, CUSAT for the excellent guidance and valuable suggestions given by them time to
time for the completion of this project.
I am extremely grateful to Dr. GEORGEKUTTY JOSEPH, HOD and our class in charge, Dr.SIBY
MATHEW, Department of Physics, Sacred Heart College, Thevara for their inspiration and
keen interest in this project. Also I would like to thank Dr.M.GEORGE, Department of
Chemistry, Sacred Heart College for his timely help and co-operation.
I express my heartfelt thanks to all members of Department of Physics, for their co-
operation and inspiration which helped me for the successful completion of this work. I also
acknowledge my sincere gratitude to the Department of Science and Technology (DST) for
the INSPIRE Scholarship for Higher Education (S.H.E).
I record my sincere thanks to all my friends who helped me directly and indirectly at various
stages of study. Especially I would like to thank my project partners – BINI JOSE T.J, NOBLE
LANCY.K and VRINDA NARAYANAN.P for their indefinite help and co-operation. I also thank
my parents for their support.
Above all I thank God for His blessings showered on me all through my life particularly
during this project.
KEVIN.V.ALEX

7. PREFACE
Transparent Conductive Oxides are wide band-gap materials with high electrically
conductivity of the order of 104
ohm-1
cm-1
, high optical transmittance (> 80%) in the
visible region and high IR reflectivity. Non-stoichiometric and doped films of oxides of
tin, indium, cadmium, zinc etc can be deposited by numerous techniques and their
properties can be suitably enhanced by controlling the deposition parameters. They
are widely used in a variety of applications such as solar cells, gas sensors, protective
coatings, heat reflectors, flat panel displays etc. Due to the high cost and less
availability of Indium which is the principal material in tin-doped indium oxide (ITO),
impurity-doped Zinc Oxide is emerging as a suitable substitute due to its tremendous
properties.
In the present study, we have prepared aluminium-doped ZnO using Spray Pyrolysis
technique. It is widely favoured deposition method because of its simplicity, low-cost,
minimal waste production and can be used for large area film coating. Studies were
done by changing aluminum doping in ZnO and deposition parameters such as spray
rate and growth temperature. The whole study consists of four chapters.
Chapter 1 is brief introduction to TCOs, its properties, various deposition techniques,
Zinc Oxide TCO and its applications. Chapter 2 gives a short description about the
spray pyrolysis setup and various characterization tools such as Stylus profiler, Source
measuring unit (SMU), UV-Visible spectrophotometer, X-ray diffractometer and
Scanning electron microscope (SEM). It also includes the preparation of Al:ZnO films.
Chapter 3 discusses the results and analysis of our work. It consists of two different
studies – effect of Al-doping in ZnO and growth rate/growth temperature study.
Chapter 4 gives the general conclusion and brief coverage of the future scope of the
work.

8. CONTENTS PAGE NO.
CHAPTER 1 - INTRODUCTION
1.1. Transparent Conductive Oxides 1
1.1.1. Theory of conductivity and transparency 2
1.1.2. Electrical Properties 4
1.1.3. Optical Properties 5
1.1.4. Thermal and Chemical Stability 6
1.1.5. Figure of Merit 6
1.2. Thin film deposition methods 7
1.2.1. Physical vapour deposition 7
1.2.2. Chemical vapour deposition 8
1.3. Zinc Oxide TCO 9
1.3.1. Properties of ZnO 9
1.3.2. Applications of ZnO as TCO 10
CHAPTER 2 – EXPERIMENTAL SECTION
2.1. Spray Pyrolysis setup 12
2.2 Characterization Tools
2.2.1. Stylus Profiler – Measurement of thickness 13
2.2.2. Determination of resistivity by two probe method 14
2.2.3. UV-Visible Spectrophotometer- Absorption & Transmission Spectra 14
2.2.4. X-ray diffraction studies 16
2.2.5. Scanning electron microscope (SEM) 17
2.3. Preparation of ZnO TCO 18

9. CHAPTER 3 – RESULTS & ANALYSIS
3.1. Aluminium doping in ZnO 19
a. Structural analysis 19
b. Electrical and Thickness analysis 20
c. Optical studies 22
d. Scanning electron microscope (SEM) 24
3.2. Growth rate / Growth temperature study 25
a. Structural analysis 25
b. Optical studies 26
c. Electrical and thickness analysis 28
CHAPTER 4 – CONCLUSION 29
References 30

10. 1
Chapter-1
Introduction
1.1] Transparent Conductive Oxides (TCO)
Transparent Conductive oxides (TCO) are remarkable materials with high electrically
conductivity of the order of 104
ohm-1
cm-1
and high optical transparency(> 80%) in
the visible region of electromagnetic spectrum. The common TCOs that are being
currently studied are – CdO, indium tin oxide (ITO), fluorine doped tin oxide (FTO),
impurity doped ZnO, etc. Various thin film deposition techniques such as dc
sputtering, plasma enhanced chemical vapour deposition, spray pyrolysis, etc are used
for TCO preparation. The research on thin film TCOs has received greater attention
due to their tremendous applications in modern science and technology. Various
TCOs are integrated in most of the opto-electronic devices such as flat displays,
photovoltaic cells and also used as gas sensors, heat reflective mirrors etc [9,10]. A
thin material created by layers of atoms/molecules/ions/cluster of species by
condensation process can be termed as thin film growth. This process involves
deposition of a thin layer of required material on to a backing material termed
substrate. Substrates are selected as per the required applications and thin film growth
conditions. Quartz, glass, Si, sapphire etc. are few examples of substrates that are
being used for various thin film deposition. [1, 2, 3]
TCOs are wide band gap materials with greater carrier concentration in the conduction
band that arise from the crystal defects or dopant impurities. The higher concentration
will cause absorption of electromagnetic radiations. Thus a TCO maintains a
compromise between electrical conductivity and optical transmission. [1, 2, 3]
The properties of the TCOs are determined by the following parameters-
1. Method of deposition
2. Substrate material &growth temperature
3. Background pressure
4. Rate of deposition
5. Precursor solution

11. 2
1.1.1) Theory of conductivity and transparency
The equation of an electromagnetic (EM) wave travelling along the positive z-
direction in a conducting medium is given by
Ex=E0 exp(-ωµζ/2)1/2
z expi[ωt-(ωµζ/2)1/2
z] ……….(1)
where ‘µ’ is the permeability of the medium
‘ζ’ the conductivity of the medium
‘ω’ the angular frequency of the wave
The presence of z in the power of the external term indicates that the amplitude of the
wave gets attenuated within a distance inside the conductor. ‘Skin depth’ is defined as
the distance in the conductor at which the electric field vector of the EM wave in the
medium decays to (1/e) times of its value at the surface.
i.e, the distance, δ = (2/ωµζ)1/2
……….(2)
is called the skin depth or penetration depth. It is clear from the equation that skin
depth is inversely proportional to the frequency of the EM wave. Thus a conductor
can act as a shield for high frequency E.M waves and for a perfect conductor (ζ = ∞),
‘δ’ becomes zero.
But for a perfect insulator, ζ = 0 and therefore the value of skin depth becomes
infinite. Thus an insulator is transparent to electromagnetic waves.
Reflection and transmission of electromagnetic waves
Now, consider an EM wave incident normally on an infinite plane boundary
separating two media of impedances Z1 and Z2.The impedance to E.M waves is
defined as the ratio of the instantaneous electric field intensity to the instantaneous
magnetic field intensity and for a dielectric medium, it is given by
Z = (µ/ε)1/2
……….(3)
where ‘µ’ and ‘ε’ are the permeability and permittivity of the medium. Similarly, the
impedance of a conducting medium is a complex quantity and its real part is given as
Z = (ωµ/ζ)1/2
……….(4)
The reflection coefficient is given by
R = ER/EI = (Z2 – Z1)/ (Z2 + Z1) ……….(5)
and the transmission coefficient is given by

12. 3
T = ET/EI = 2Z2/ (Z2 + Z1) ……….(6)
where EI,ER,ET represents the electric field vectors of the incident, reflected and
transmitted waves.
Case 1 - If the wave is travelling in air and strikes a perfect conductor of ζ = ∞ and
therefore Z2 = (µω/ζ)1/2
=0 at normal incidence, then
R = ER/EI = (Z2- Z1)/ (Z2 + Z1) = -1 ……….(7)
EI = -ER
i.e, the wave is totally reflected.
Similarly, T = 2Z2/ (Z2+Z1) = 0 ……….(8)
i.e, there is no transmission.
Thus good conductors are very good reflectors of electromagnetic waves.
Case 2 - For two dielectric media, the values of reflected as well as transmitted energy
coefficients can be expressed in terms of their refractive indices (say n1& n2). If an
E.M wave travels through free space and incidents on a dielectric medium normally,
then
Reflected energy coefficient, RE = ERHR/EIHI
= (n1 – n2/n1 + n2)2
……….(9)
Transmitted energy coefficient,TE = ETHT/EIHI
= 4n1n2/ (n1 + n2)2
……….(10)
For air-glass interface n1=1 and n2=1.5, we have
RE = (0.5/2.5)2
= 4% and TE = 6/6.25 = 96%
i.e, most of the light is transmitted. Thus insulators have high optical transmittance.
To summarize, conductivity and transparency are antonyms to each other. i.e, a good
conductor reflects E.M waves while insulators are transparent to them. The
conductivity and transparency of semiconductors lie in between that of conductors and
insulators. [4,5]
Partial transparency and fairly good conductivity may be obtained in thin films of a
variety of metals. The only way to obtain good transparent conductors is to create
electron degeneracy in a wide band gap (greater than 3 eV) oxide by controllably
introducing non-stoichiometry or appropriate dopants. These conditions are very

13. 4
conveniently obtained in oxides of cadmium, tin, indium, zinc and their alloys in thin
film form, prepared by a number of deposition techniques.[1]
Beer-Lambert’s Law
For the photon absorption process to occur, the incident photon energy must be equal
to or greater than the band gap energy of the semiconductor. The threshold or upper
cut-off wavelength is given by the equation
λg = hc/Eg ……….(11)
Incident photons with wavelengths lower than λg become absorbed as they travel
through the semiconductor and the intensity of the light decays exponentially with
distance into the semiconductor. The light intensity at a distance x from the surface is
given as
I = I0exp(-αx) ……….(12)
where I0 – intensity of the incident wave
α – absorption coefficient
When I = I0 / e, x = δ = 1/α is termed as penetration depth which is defined as the
distance from the surface of the semiconductor at which the incident light intensity
drops to its (1/e)th
value. [6]
Extinction Coefficient – It is the imaginary part of the refractive index of a medium
which denotes the measure of attenuation of the E.M wave while propagating through
it. Mathematically, it is given as
κ = αλ/4π ……….(13)
where ‘λ’ is the wavelength of the electromagnetic beam. [6,7]
1.1.2) Electrical Properties
Most of the transparent conducting oxides are n type with a high carrier concentration
of about 1018
– 1021
cm-3
and a low mobility of about 1-50 cm2
V-1
s-1
.The manufacture
of p-type TCOs are tedious because during acceptor formation, the acceptor states
(holes) are localized on oxygen ions leading to lesser conductivity. Stoichiometric
crystalline TCOs are good insulators.
The conductivity of a TCO is given as σ = neμ ,i.e , conductivity depends on both
carrier concentration as well as mobility.The high conductivity of TCO is mainly due

14. 5
to the presence of anion (oxygen) vacancies or excess interstitial cations (metal ions)
which act as electron donor sites. The electrical properties of TCOs can be improved
by the addition of suitable dopants and it should be noted that effective doping effects
are obtained only when the ionic radius of the dopant is the same as or smaller than
that of the host ion. The replacement of the host cation (eg: Zn2+
) by a higher valency
cation (eg: Al3+
) in the oxide films increases the n-type conductivity, while that by a
lower valency cation produces a hole which acts as a trap in the n-type semiconductor
and decreases the conductivity. Similar effects occur if the anion sites are doped with
higher or lower valency impurities.
Mobility is a key parameter which can enhance both conductivity and transmission
and it is given by the relation μ = eτ/m* where ‘η’ is the mean time between collisions
and ‘m*’ is the effective electron mass. Increasing the carrier concentration will
degrade the transmittance while increase in mobility has no negative effect in its
properties. The low mobility in TCOs appears to be due to the decrease in mean time
between collisions as a result of the coulomb interaction between electrons and
dopants. Mobility becomes almost temperature independent at higher carrier
concentrations. It is not possible to independently increase carrier concentration and
mobility in practical TCOs with relatively higher carrier concentrations. [1, 2, 3]
1.1.3) Optical Properties
Optical properties of TCOs are determined by its refractive index, extinction
coefficient band-gap and geometry of the film. The transmission spectrum of
transparent conductive oxides shows high optical transmittance in the visible and
near-IR regions of the solar spectrum and it is a direct consequence of their wide band
gap (Eg ≥ 3.0 eV). Their fundamental absorption edge generally lies in the UV region
and shifts to shorter wavelengths with increasing carrier concentration (doping). This
shift is due to the filling of the states near the bottom of the conduction band and is
well known as the Moss-Burstein shift.
The cut-off wavelength above which a TCO shows high reflectance towards the
incident infrared radiations is called the plasma wavelength. In terms of frequency, the
minimum frequency above which a TCO becomes transparent to electromagnetic
waves is termed as plasma frequency. For frequencies greater than the plasma
frequency, the electrons cannot respond to the fluctuating electric field of the incident
wave and thus the material becomes transparent.The plasma frequency is given by the
equation,
ωp= (4πne2
/m*)1/2
……….(14)

15. 6
where ‘n’ is the electron concentration and ‘m’ is the effective mass of the electron. It
is clear that the plasma frequency is directly proportional to the square root of the
carrier concentration. The transmission in the visible and NIR region is limited by
several factors
1) Reflection losses
2) Absorption in the film primarily due to charge carriers
3) Film thickness
4) Scattering due to inhomogeneties in the film (unreacted/partly reacted chemical
species)
5) Nature of the substrate [1, 2, 3]
1.1.4) Thermal and Chemical Stability
The temperature above which the properties of TCO films show degradation due to its
chemical decomposition is termed as thermal stability temperature and it is about
250o
C for ZnO. The resistivity of TCOs will increase with the rise in temperature.
Chemical stability of a TCO is determined by its ability to resist corrosive
environment and treatment. ITO films are more stable in oxidizing atmospheres while
doped ZnO shows stability in reducing atmosphere. [2,3]
1.1.5) Figure of merit
Figure of merit is a quality parameter which outlines the correlation between
conductivity and transmission of a TCO. According to Fraser and Cook, figure of
merit is defined as
F = T/RS ……….(15)
where ‘T’ is the transmission and ‘RS’ is the sheet resistance and this depends on the
film thickness. A figure of merit independent of film thickness was developed by Iles
and Soclof which is given by the relation
F = - RSln(T) = α/σ ……….(16)
Where ‘α’ and ‘ζ’ are the absorption coefficient and conductivity of the film
respectively. [3]

16. 7
1.2] Thin film Deposition Methods
The properties of thin films strongly depend on the deposition methods. The following
steps are involved in the deposition techniques-
1. Generation of atoms/molecules from the source
2. Transport of the source atoms/molecules to the substrate
3. Deposition of the atoms/molecules on the substrate
4. Post growth treatment and analysis.
The thin film deposition methods can be broadly classified into two-
1. PhysicalVapourDeposition (PVD)
In physical deposition techniques, atoms/molecules of the source material are
vaporized by physical process such as thermal evaporation or bombardment by high
energy particles. Some of the major physical techniques are detailed below.
A) Vacuum Evaporation
It is a PVD technique in which the material is thermally vaporized from a source and
deposited on the substrate in a vacuum environment without collision with the gas
molecules present in the space between the source and substrate. It is usually
conducted in the pressure range 10-5
to 10-9
Torr.
B) Pulsed Laser Deposition (PLD)
In this method, a laser pulse is focused into the source in a vacuum chamber. The laser
pulse locally heats and vaporizes the target surface ejecting atoms/molecules which is
deposited on the substrate.
C) Sputtering Process
Sputtering is the most widely used PVD technique for a large area coating in which a
high energy particles strike the surface to dislodge the atoms form the surface of the
source material. The ejected or sputtered atom is allowed to condense on a substrate to
form the thin film.

17. 8
2. ChemicalVapour Deposition (CVD)
In chemical vapour deposition, a chemical reaction of one or more vapours species
incident on the substrate produces the thin film of desired composition. The main
CVD techniques are discussed below.
A) Spin Coating
It is a simple and commonly used technique for deposition of thin films on wafer
surfaces. In this method, an excess amount of solution is placed on the substrate which
is then allowed to rotate at high speed about a perpendicular axis to the substrate. The
solution uniformly spreads over the substrate due to centrifugal force and thus the film
is obtained.
B) Dip Coating
This method involves the process of inserting the substrate into a solution of
hydrosable metal compounds and pulling it out at constant speed under controlled
temperature and pressure.
C) Spray Pyrolysis
Spray Pyrolysis is one of the most preferred chemical deposition techniques due to its
simplicity, low cost, minimal waste production and can be employed for large area
film coating. This method involves the spraying of solution (source) onto the
substrate, which is maintained at required temperature. The sprayed droplets called the
aerosol undergo a pyrolytic (endothermic) decomposition and thus the thin film is
formed over the substrate. The remaining volatile by-products and the excess solvent
in the gas phase are pushed out of the spray chamber through the exhaust. The quality
of the film produced depends on the parameters like spray rate, substrate temperature,
background pressure, precursor concentration and the carrier gas pressure.
The main steps involved in Spray Pyrolysis can be branched into three –
1) Atomization of the precursor solution –In this process, the droplets are generated
from the precursor solution, which is sprayed onto the hot substrate. It must be noted
that the precursor should not react with the solution. The atomizer employed
determines the droplet size, spray rate and the initial velocity of the droplets.
2)Transport of the solutiondroplets – The movement of the sprayed droplets depends
on the initial velocity which is determined by the atomizer.Droplets should not form
powder or salt particles while being transported to the substrate.
3)Decomposition of the precursor on the hot substrate – When the droplet strikes the
hot substrate evaporation of residual solvent, spreading of droplets and the

18. 9
decomposition process occurs simultaneously. The growth temperature selected must
be higher than the decomposition temperature of the Precursor. [2, 3]
1.3] Zinc Oxide TCO
Zinc Oxide is a binary compound semiconductor which exists in the wurtzite, zinc
blende or rock salt crystal structure depending upon the growth conditions. The
wurtzite structure is thermodynamically stable at room temperatures while the rock
salt structure is obtained at high pressure. The zinc blende structure can be stabilized
only by the growth on cubic structure.Wurtzite structure has hexagonal unit cell with
lattice parameters a and c in the ratio (c/a) = 1.66.The structure composes of two
hexagonally close packed (hcp) sub-lattices and each sub-lattice consists of four atoms
per unit cell. Every oxygen atom lies within the tetrahedral group of four zinc atoms
or vice-versa and thus has tetrahedral cordination. Doping of ZnO thin films with Al,
Ga etc will enhance its electrical conductivity and also shows a higher transmittance
in the visible region. [2]
Figure 1.1 - Wurtzite structure Figure 1.2 - Zinc blende structure
1.3.1) Properties of ZnO
 Direct and wide band gap – Its band gap is about 3.44eV at low temperatures
but it is 3.31eV at ordinary temperatures. This enhances its application in
optoelectronics in the visible and UV region.

19. 10
 Large exciton binding energy – The free exciton binding energy of ZnO is 60
meV.This clearly shows that efficient excitonic emission in ZnO can persist at
room temperature and higher.
 Strong luminescence – It is used as a phosphor material due to the strong
luminescence in the green region of the spectrum.
 Strong sensitivity of surface conductivity to the presence of adsorbed species –
Conductivity of ZnO thin films is very sensitive to the exposure of the surface
to various gases. Therefore it can be used as a cheap sensor/detector.
 Amenability to wet chemical etching – It has been reported that ZnO thin films
can be etched with acidic, alkaline and mixture solutions.(for example ZnO can
be easily etched using diluted sulphuric acid).
 High thermal conductivity –ZnO is used as an additive. (eg: ZnO is added to
rubber inorder to increase its thermal conductivity). [2]
1.3.2) Applications of ZnO as TCO
The optical & electrical properties as well as the abundance and low cost of ZnO
make it as one of the most promising material for TCO. The major practical
applications of ZnO thin film conducting oxides are described below.
1) Solar Cells– Thin film oxide-based solar cells have received much appreciation in
recent years due to its competent power conversion efficiency, low resistance and
easily fabrication. It is generally used as a front contact/window layer inthin film solar
cells and can also function as an anti-reflection coating.
2) Gas Sensors – It can be used as a sensor because of its high sensitivity to chemical
environments which is governed by the oxygen vacancies on the surface. The
conductivity is lowered during oxidation while it increases during the reduction
process. During oxidation, electrons are withdrawn from the vacancy sites by the
adsorption of oxidizing molecules leading to a decrease in conductivity. On the other
hand, reducing molecules can react with surface-adsorbed oxygen, leaving behind an
electron and hence higher conductivity.
3) Thin Film Transistors (TFT)–A TFT is a field-effect transistor that is built by
depositing thin films of semiconducting material as well as the dielectric layer and the
metallic contacts on a glass substratewhich is widely used in liquid crystal displays

20. 11
(LCD).Due to its higher conductivity and transparency, ZnO can be used as the
semiconducting material in TFT’s.
4) Heat mirrors–Since TCO films show high reflectance in the IR region, we can
effectively use it as heat mirrors. The commercially available TCO films have a
plasma wavelength in the range 1000-1500 nm.
5) UV absorber – The fundamental absorption edge of TCO films lie in the UV region
and it shows higher absorbance in this region. So it can be also used as an UV
absorber. [1, 2, 3]

21. 12
Chapter-2
Experimental Section
2.1]Spray Pyrolysis Setup
In this project, the thin film deposition was done using a custom designed computer
controlled Spray Pyrolysis Equipment. As discussed earlier, deposition using spray
pyrolysis method involves the spraying of precursor solution over a substrate kept at
required temperature, where the aerosol further undergoes decomposition over the
substrate to obtain required thin film material. This technique has wide industrial level
application. The setup for spray pyrolysis is shown below.
Figure 2.1-Spray Pyrolysis setup
It mainly consists of
a) Spray nozzle – The spray nozzle is connected to the carrier gas (N2) as well as
the precursor solution.Precursor is pumped using a metering unit, using which
the rate of spray is controlled. On applying a fixed pressureof carrier gas, low

22. 13
pressure is created at the tip of the nozzle and the precursor spay (aerosol) is
generated at the tip of the nozzle.
b) Rotor for spray nozzle – A microprocessor controlled stepper motor is used for
the motion of the nozzle for spraying over the entire area of the substrate.
c) Heating plate–A coil act as the heating element to which the molten tin bath
heating substrate is attached. The temperature of the heating plate is monitored
by means of a temperature controller.
d) Substrate– The substrate is placed over the substrate heater. In the present work
3 mm float glass is used as the substrate.
e) Gas regulator valve – It is used to control the flow of the carrier gas through
the nozzle. Nitrogen was used as the carrier in this study.
f) Air tight chamber–The whole apparatus is mounted inside an air tight chamber
which isolates the system from the external environment. An exhaust pump is
connected to the chamber for removing the gases produced during the process
of thermal decomposition. In the present work, the spray pyrolysis experiment
was carried out at an inert gas atmosphere (N2) of know constant pressure.
2.2] Characterization Tools
2.2.1) Stylus Profiler – Measurement of thickness
A portion of the sample is etched using diluted sulphuric acid inorder to remove the
film which creates a step on the sample surface. The Stylus Profiler consists of an
electromechanically movable diamond tipped pointer which is moved across the step.
The difference in the reading between the coated and etched regions gives the film
thickness. The average value of the thickness is evaluated by varying the scan length.
Thickness was measured from various regions of the sample and average thickness is
determined. [3]
Figure 2.2- Stylus Profiler

23. 14
2.2.2) Determination of Resistivity by two-probe method
The resistivity measurement of the samples was done by the two-probe method using
highly conducting silver paste as electrodes. The current-voltage measurements of the
samples were carried out in a Keithley’s source measuring device (SMU). The current
for different values of applied voltages were measured for a sample and the average
value of resistance is evaluated by applying Ohm’s law. Using the relation ρ=RA/L,
resistivity of the film is determined.[3]
where R - average value of measured resistance of the sample
A -Area of the film
L – Spacing between the electrodes
2.2.3) UV-Visible Spectrophotometer – Absorption & Transmission spectra
The basic working principle of a UV-Visible spectrophotometer is that it compares the
transmitted light intensities of the sample and reference and give as the required data.
Mathematically,
Transmission %, T = (I/I0)*100 ……….(17)
where I & I0 are the transmitted as well as incident light intensity. Absorbance (A) is
defined as the negative logarithm of the transmission percentage.
A = -ln(I/I0) ……….(18)
Figure2.3-Setup of UV-Visible spectrophotometer

24. 15
The basic configuration of a spectrophotometer consists of –
a) Light Source – The commonly used light sources are halogen lamps/ deuterium
lamps/ xenon lamps etc.
b) Monochromator – It functions as a filter which allows only a single wavelength of
light reaches the sample at a time.
c) Beam Splitter – It is used to split the beam into two and thus allowed to pass
through the reference and sample.
d) Sample Compartment – It consists of square cell holders in which the samples and
reference are placed.
e) Detector– The light beam which enters through the sample compartment enter the
detector which is the last element of the spectrophotometer. Photomultipliers and
silicon diodes are the typically used detectors.
In this project, the absorbance and transmittance data were determined by using
Shimadzu UV-Visible spectrophotometer.
Determination of Band-gap energy
The relation between the absorption coefficient (α) and the band gap of the material is
given as
αhν= K(hν– Eg)r
……….(19)
where r = 1/2 for allowed direct transitions, r = 2 for allowed indirect transitions, r =
3 for forbidden indirect transitions and r = 3/2 for forbidden direct transitions. ‘K’ is
the parameter which depends on the transition probability.
According to Beer-Lamberts law, I = I0exp(-αt)
Therefore, α = -ln(I/I0)/t
= A/t ……….(20)
where‘t’ is the sample thickness.
Also, photon energy, E = hc/eλ = (1242.375/λ) eV ……….(21)
For direct transition, the plot between (αhν)2
vshν will show a linear dependence and
the intercept on the energy axis at (αhν)2
= 0 is measured as the band gap energy of
the material. [2, 3, 7, 8]

25. 16
2.2.4) X-ray diffraction Studies
X-ray diffraction (XRD) studies were carried out to study the crystallographic
properties of the thin films since its electrical and optical properties are influenced by
its crystallographic nature.The basic law involved in the diffraction method is the
Braggs law.
When monochromatic X-rays are incident on a crystal, the crystal lattice will act as a
series of reflecting planes. Constructive interference will occur when the path
difference betweentwo reflected waves from two different planes are an integral
multiple of λ. This condition is called Bragg’s law and is given by the relation,
2dSinθ=nλ ……….(22)
where‘n’ is the order of diffraction, ‘λ’ is the wavelength of the X-rays, ‘d’ is the
inter-planar spacing and ‘θ ’is the glancing angle.
X-ray diffractometer consists of three basic components - an X-ray tube, sample
holder and X-ray detector. X-rays are generated in a cathode ray tube by heating a
filament to produce electrons and bombarding the target material (eg-Copper) with
electrons. X-rays are produced when the electrons have sufficient energy to remove
inner shell electrons from the target material. These X-rays are then collimated and
directed onto the sample. The intensity of the reflected rays is recorded as the sample
and detector are rotated.
Figure 2.4- Setup of X-ray diffractometer

26. 17
Experimentally obtained diffraction patterns of the sample are compared with the
standard powder diffraction files published by the International Centre for Diffraction
Data (ICDD).The average grain (L) size of the film can be evaluated using the Debye
-Scherer’s formula given by
L = 0.9λ/βcosθ ……….(23)
where ‘λ’ is the wavelength of the X-ray and ‘β’ is the full width athalfmaximum
intensity in radians.In this project, the diffraction measurements were done using the
Philips X’pert x-ray diffractometer. [2, 3, 7]
2.2.5) Scanning electron microscope (SEM)
The surface morphology of the prepared samples can be studied by analyzing the
SEM images. Scanning electron microscope uses electrons instead of light to produce
high resolution images. It mainly consists of a filament which acts as the cathode from
which electrons are ejected out and an anode which is positive with respect to the
filament. The electrons that are accelerated towards the anode is focused into the
sample and due to the impact, secondary electrons are produced. These electrons are
collected by a secondary detector. [2,3]
Figure 2.5- Setup of scanning electron microscope
In this project, the surface morphology was studied using Tescan Scanning electron
microscope.

27. 18
2.3]Preparation of ZnO TCO
Substrate Cleaning –In this project, we have used the glass of a peculiar brand
named ‘Modiguard’ (3 mm thick) inorder to deposit ZnO thin films. The substrates
(glass) werefirst cleaned with dilute acetic acid and then by soapsolution. After this,
substrates were washed in tap water. The cleaned substrates were then rinsed using
alcohol,dried and placed in the chamber for film coating.
Sample Preparation – Two different studies were conducted to (1) optimize the Al
doping concentration in ZnO and (2) and to reduce the growth time.
1) Aluminium doping in ZnO: – A 100 ml, 0.3M precursorsolution was prepared by
adding 6.4042 g of zinc acetate dihydrate and varying aluminum acetylacetonate
concentration by 0, 1, 2, 2.5, 2.75, 3& 3.5%in the precursor solution using methanol
as solvent.The substrate temperature and growth time was set to 4900
C and 50 min.
respectively. The spray rate was kept at 2 ml/min while the carrier gas pressure was
0.5 bar.
2)Growth rate/ growth temperature study –In this study the [Al/Zn] ratio in the
precursor was kept constant as 2.75 %. Here the substrate temperature was varied by
4900
C, 5050
C, 5200
C.The growth time and rate of spray was kept at 20 min. and 5
ml/min respectively.

28. 19
Chapter-3
Results and discussion
3.1] Aluminium doping in ZnO
a) Structural analysis
From the previous work, we fixed the growth temperature at 490 o
C and Al doped
ZnO (Al:ZnO) thin films were deposited by varying the Al/Zn concentration in the
precursor solution in the range 0 to 3.5 %. Fig.3.1 shows the X-ray diffraction pattern
recorded from Al doped ZnOthin films with 2θ in the range 20 to 80o
. X-ray
diffraction peaks corresponding to the wurtzite phase of ZnO are identified and
indexed as per the JCPDS file: 897102.The obtained Al:ZnO thin films were
preferentially oriented along (002) plane.
Figure. 3.1: X-ray diffraction pattern recorded from Al:ZnO (Al/Zn – 0 to 3% nominal)

29. 20
Table 1 shows the variation of FWHM of the preferentially oriented peak (002) of
Al:ZnO and average grain size of the samples. It is clear from the table 1 that the
FHWM decreased from 0.433 to 0.236 on increasing the Al doping concentration (in
the solution) from 0to 3%.This shows that the crystalline quality of ZnO is improved
upon Al incorporation into ZnO lattice. The average grain size of Al:ZnO increased
from 19 nm to 36 nm on increasing the Al doping concentration (in the solution) from
0 to 3%.
[Al/Zn] con.
in precursor
2θ value of (002)
plane
(degree)
FWHM of
(002) plane
(degree)
d002 spacing
(Å)
Average
grain Size
(nm)
0 % 34.393 0.4333 2.607 19
1 % 34.494 0.393 2.600 21
2 % 34.505 0.472 2.599 17
2.5 % 34.389 0.393 2.607 21
2.75 34.381 0.236 2.608 36
3 % 34.577 0.236 2.594 36
Table 1: The variation of FWHM of the preferentially oriented peak (002) of Al:ZnO and average grain size
of the samples
b) Electrical and Thickness analysis
Figure 3.2 depicts the variation of resistivity of Al:ZnO deposited with various
[Al/Zn] dopant concentrations. The increase in Al dopant concentration in the
precursor solution resulted in reduction of resistivity/sheet resistance of Al:ZnO
reaching a minimum value for a Al doping concentration of 3%. The decrease in
resistivity of Al:ZnO on increasing Al doping concentration is attributed to the
increase in carrier concentration due to the replacement of Zn2+
sites in ZnO lattice by
Al3+
ions. The figure 3.3 shows the sheet resistance mapped from Al:ZnO (Al 3%
nominal) over an area of 15 cm x 15 cm, exhibiting the uniformity of the sample.

30. 21
Figure 3.2: Variation of resistivity of Al:ZnO with Al doping concentration
Figure 3.3:Sheet resistance mapped from Al:ZnO (Al 3% nominal) over an area of 15 cm x 15 cm
Thickness of the Al:ZnO thin films were measured using Stylus thickness profiler and
found to be around ~ 1500 nm. No considerable variation in thickness is observed on
Al doping.

31. 22
[Al/Zn] con. in
precursor
Thickness
(nm)
Sheet
Resistance
(Ω/Sq)
Resistivity
(Ω-cm)
0 % 1530 ± 38 8600 ± 430 1.31 ± 0.1
1 % 1500 ± 37 213 ± 11 (3.2 ± 0.3) X 10-2
2 % 1485 ± 37 52 ± 3 (7.8 ± 0.7) X 10-3
2.5 % 1470 ± 36 33 ± 2 (4.9 ± 0.4) X 10-3
2.75% 1510 ± 38 30 ± 2 (4.5 ± 0.4) X 10-3
3 % 1550 ± 38 12 ± 1 (1.8 ± 0.2) X 10-3
3.5% 1480 ± 42 240 ± 12 (3.6 ± 0.3) X 10-2
Table 2: Variation of thickness of Al:ZnO (Al 0 to 3.5 %)
c) Optical studies
Figure 3.4 depicts the transmission spectra recorded from Al:ZnO thin films (Al- 0 to
3%). The optical transmittance study performed on the Al:ZnO thin films exhibited an
interesting result. The un-doped sample (Al:ZnO - 0% Al) exhibited a broad
absorption edge due to the presence of defects in the film (Zn rich and/or O deficient
ZnO). Replacement of Zn2+
sites by Al3+
ions in ZnO lattice, on doping ZnO with Al
reduced the Zn content in the Al:ZnO film favouring the growth of transparent
Al:ZnO. On increasing the doping concentration above 1%, the transmittance showed
a decrease due to the increased light absorption by free charge carriers. We can also
observe a blue-shift in the fundamental absorption edge. Correspondingly, an
enhancement in optical band bap was also exhibited by Al:ZnO on increasing the
Al/Zn doping concentration. ZnO exhibited a band gap of 3.24eV (figure 3.5) in
comparison with that of 3% Al doped Al:ZnO (3.40eV). The enhancement of band
gap is attributed to the Burstein-Moss (BM) shift, due to the increase in carrier
concentration. The BM shift is the phenomenon in which the apparent band gap of a
degenerate semiconductor is enhanced due to the highly populated states near the
conduction band edge. It is normally observed when the doping concentration exceeds
the density of states of the conduction band edge. [1,2]

32. 23
Figure 3.4: Transmission spectra of the undoped and Al doped (1% & 3%) ZnO samples.
Figure. 3.5: (αhν)2
versus hνplot of ZnO and Al:ZnO(Al 3% nominal)

33. 24
Figure3.6 represents the comparison between 3% Al:ZnO and commercially available
fluorine doped tin oxide (FTO). Thus the study conducted here shows that by using
spray pyrolysis method one could attain Al:ZnO with electrical and optical properties
comparable with that of commercial FTO.
Figure 3.6: A comparison between 3% Al:ZnO and commercially available fluorine doped tin oxide (FTO).
d) Scanning electron microscopy analysis
Finally, to study the surface morphology of the obtained Al:ZnO thin films, SEM
images were recorded. Figure 3.7 shows the SEM image of Al:ZnO (Al-3%). We can
observe regular surface morphology with granular uniform grains.
Figure. 3.7: SEM image of Al:ZnO (Al - 3 % Al)

34. 25
3.2] Growth rate/Growth temperature study
In order to reduce the growth time, we intentionally increased the rate of spray to 5
ml/min and fixed the [Al/Zn] concentration as 2.75% in the precursor solution, while
keeping all other parameters constant as our previous study section 3.1. Equal quantity
of precursor solution (100 ml) was sprayed for a growth time of 20 minutes. On
increasing the rate of spray from 2 to 5 ml/min. the film growth rate increased from
30.2 to 76.5 nm/min. respectively. Interestingly the Al:ZnO (Al-2.75%) exhibited a
resistivity of about 7.65 x 10-3
ohm cm that is comparable with that prepared with a
rate of spray of 2 ml/min (4.53 x 10-3
ohm cm ). However, the resistivity of the sample
prepared with higher rate of spray of 5 ml/min. exhibited a increasing tendency with
time, while the resistivity of the sample prepared at a lower rate of spray (2 ml/min)
was stable. The observed behavior of increase in resistivity by Al:ZnO (Al-2.75%)
prepared at 5 ml/min., is attributed to the adsorption of atmospheric oxygen by the
sample. As we maintained the pressure inside the chamber constant for both the
experiment (2 and 5 ml/min.), the quantity of oxygen present in the chamber was
apparently similar. Thus the sample, which was prepared with a rate of spray 5ml/min,
was formed with high oxygen deficiency that resulted in the instable resistivity of the
sample.
Further, to study the effect of growth temperature, a set of samples was prepared with
various growth temperatures 490, 505 & 520o
C, while maintaining all other growth
parameters constant.
a) Structural analysis
Figure 3.7 shows the X-ray diffraction pattern of Al:ZnO(Al - 2.75%) prepared at
various growth temperatures 490,505,5200
C. The samples are preferentially oriented
along the (002) plane. The obtained XRD pattern is compared with the standard data
confirming the formation of ZnO in hexagonal (wurtzite) phase. On increasing the
growth temperature, no considerable variation in crystalline quality or average grain
size of the sample was observed (Table 3).

35. 26
Figure 3.8: X-ray diffraction pattern of 2.75% Al:ZnO prepared at various growth temperatures 490, 505 &
520 0
C
Growth
temperature
(o
C)
Peak Postion
(2θ) of (002)
plane
(degree)
FWHM of
(002) plane
(degree)
d002 spacing
(A0
)
Grain size
(nm)
490 34.497 0.3936 2.599 21
505 34.453 0.3936 2.603 21
520 34.477 0.3936 2.601 21
Table 3: The variation of FWHM of the preferentially oriented peak (002) of Al:ZnO and average grain size
of the samples
b) Optical studies
Figure 3.9 shows the transmission spectra of Al:ZnO (Al – 2.75 %) deposited at
various growth temperature. No considerable variation in optical transmittance is
observed increasing the growth temperature.

36. 27
Figure 3.9: Transmission spectra of Al:ZnO (Al – 2.75 %) deposited at various growth temperature
The optical band gap of the samples were determined from (αhν)2
versus hν plot and
are 3.33, 3.34 and3.34eV for samples grown at 490, 505 and 520 o
C respectively.
Figure 3.10: (αhν)2
versus hνplot of Al:ZnO(Al - 2.75% nominal) at various growth temperature 4900
C and
5200
C

37. 28
c) Electrical and Thickness analysis
Figure 3.11: Variation of resistivity of Al:ZnO with growth temperature
The resistivity of the Al:ZnO thin films deposited at various growth temperature were
measured. It is observed that on increasing the growth temperature from 490 to 520 o
C
the resistivity of the Al:ZnO increased (figure 3.11). Table 4 shows the variation in
sheet resistance with growth temperature. However the increase in growth temperature
resulted in the increase of growth rate from 76.5 nm/min to 92.1 nm/min which is
evident from the increase in film thickness (Table 4). The increase in rate of growth
may result in the formation of Al:ZnO thin films with instable resistivity and inorder
to attain stability, the film adsorbs atmospheric oxygen which further increases its
resistivity. This study showed that higher rate of spray and growth temperature results
in the formation of unstable Al:ZnO, which is attributed to the growth of oxygen
deficient Al:ZnO thin films.
Growth
temperature
(o
C)
Thickness
(nm)
Sheet Resistance
(Ω/Sq)
Resistivity
(Ω-cm)
490 1531 ± 38 412 ± 21 (6.3 ± 0.5) X 10-2
505 1634 ± 41 514 ± 26 (8.4 ± 0.7) X10-2
520 1842 ± 46 694 ± 35 (1.3 ± 0.1) X10-1
Table 4: Variation of thickness of Al:ZnO at various growth temperatures

38. 29
Chapter-4
Conclusion
Aluminium doped ZnO transparent conductive films were prepared on glass substrate
on an area of 15 cm x 15 cm by varying the Al doping concentration and growth
temperature using Spray Pyrolysis method. The structure of the films was studied
using XRD analysis and confirmed the formation of ZnO with hexagonal wurtzite
crystal structure. The deposited films exhibited excellent crystalline structure with
(002) preferential orientation. The grain size was estimated for each sample. The
thickness of the film was measured using the Stylus profilometer and found to be in
the range of ~ 1500 nm. The sheet resistance of the samples was measured by using a
source-measuring unit and the resistivity of each sample is determined. The variation
in resistivity and optical band gap/transmittance of Al:ZnO was studied with the
various [Al/Zn] concentration – 0 to 3.5 % in the precursor solution. An enhancement
in optical band bap was exhibited by Al:ZnO on increasing the Al/Zn doping
concentration. ZnO exhibited a band gap of 3.24eV in comparison with that of 3% Al
doped Al:ZnO (3.40eV). The enhancement in band gap is attributed to the Burstein-
Moss shift, due to the increase in carrier concentration. The resistivity of the Al:ZnO
gradually reduced from 1.31ohm-cm to a minimum value of 1.8 x 10-3
ohm-cm on
increasing the Al concentration from 0 to 3 %. The Al:ZnO sample that exhibited
minimum resistivity of 1.8 x 10-3
ohm-cm showed a optical transmittance ≥ 65%
higher than the wavelength ≥ 410 nm. Moreover this sample exhibited a sheet
resistance between 10-20 ohm/Sq. over a large area on glass substrate. The increase in
spray rate (from 2 ml/min to 5 ml/min) and growth temperature favoured the growth
of resistive Al:ZnO films. The Al:ZnO TCOs prepared can be employed as the front
contact/window layer in solar cells, a UV absorber or can be used in gas sensors.

39. 30
References
1) Transparent Conductors – A Status Review by K.L.Chopra, S.Major and
D.P.Pandya, Dept.ofPhyscis, IIT,New-Delhi, Thin solid films, 102(1983), 1-46
2) Highly conductive and transparent ZnO thin film using chemical spray
pyrolysis technique: effect of doping and deposition parameters by Vimal
Kumar T.V, Thin-film photovoltaic division, Dept. of Physics , CUSAT
3) Characterization of Transparent Conducting thin films grown by Pulsed laser
deposition and RF Magnetron Sputtering by Manoj.R, Dept. of Physics,
CUSAT
4) Introduction to Electrodynamics by David.J.Griffit, Prentice-Hall of India,
New-Delhi (1999) , p:382-398
5) Refresher course in Physics – Volume 1- Waves,Vibrations & E.M.Theory,
Schand & Co, New-Delhi
6) Opto-electronics and Photonics: Principles and Practices , S.O.Kasap, Pearson
7) Structural and optical properties of zinc oxide thin films prepared by spray
pyrolysis method by C.Gumus, O.M.Ozkendir, H.Kavak and Y.Ufuktepe ,
Journal of Opto-electronics and advanced materials,Vol.8,No.1,February
2006,p:299-303
8) Semiconducting properties of aluminum doped ZnO thin films grown by spray
pyrolysis technique by M.A.Mahadik, Y.M.Hunge, S.S.Shinde,
K.Y.Rajpureand C.H.Bhosale, Journal of Semiconductors, Vol.36, No.3,
March 2015, 033002:1-6
9) Properties of Al-doped ZnO thin films grown by spray pyrolysis by
Shashidhara Bhat, Shrisha B.V, Gopalakrishna Naik, Scholars Research
library, Archives of Physics Research, 2013,4(4):20-27
10)Synthesis and Characterization of ZnO films on Glass substrate via Spray
Pyrolysis deposition technique, Andrea Gracia Cuevas and Alladin Caballes
Jasmin, International Journal of Technology Innovations and Research, Vol.2,
March 2013, p: 78-85

40. 31
11)Handbook of Transparent conductors – David S.Ginley, Hideo Hosono,
David.C.Paine, Springer Publication, New-York, 2010
12)Transparent conductive zinc oxide – Basics and applications in thin filmsolar
cells by Klaus Ellmer, Andreas Klein, Bernd Rech, , Springer Publication,
New-York, 2007
13)Improved transparent conductive oxides through modulation-doped
ZnO/ZnMgO thin films by David J.Cohen, 2008, p:19 – 44
14)Semiconducting Transparent thin films by H.L.Hartnagel, A.L.Dawar,
A.K.Jain & C.Jagadish, Institute of Physics Publishing, Bristol & Philadelphia,
1995